Add namespaced plugin configuration support

Plugin packages need somewhere to keep their own settings, but MVT
rewrites its config.yaml with only the fields it knows about, so any
foreign section is dropped. Add MVTPluginSettings, a pydantic-settings
base class that gives each plugin its own file under the MVT config
folder and its own MVT_PLUGIN_<NAME>_ environment variable namespace.

Settings resolve from constructor arguments, then the environment, then
the plugin file, then the field defaults. Saving skips the values the
environment currently supplies, so credentials passed as environment
variables are not copied to disk, and writes through a private temporary
file so a settings file is never partially written or briefly readable
by other users.

docs/development/custom_commands.md

@@ -56,6 +56,10 @@ pipx inject mvt my-mvt-plugin
 When MVT is installed in an active virtual environment, install the plugin with
 `pip` in that environment.
 
+Command packages that need their own settings, such as an API key, should store
+them in a namespaced [plugin configuration file](plugin_configuration.md)
+rather than in MVT's own `config.yaml`.
+
 ## Load a Command File
 
 For local commands that are not packaged, create a Python file that exports one